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Epidemiology

Tourette syndrome is found among all social, racial and ethnic groups,has been reported in all parts of the world, and is three to four times more frequent among males than among females. The tics of Tourette syndrome begin in childhood and tend to remit or subside with maturity; thus, a diagnosis may no longer be warranted for many adults, and prevalence is much higher among children than adults. Children are five to twelve times more likely than adults to be identified as having tic disorders;as many as 1 in 100 people experience tic disorders, including chronic tics and transient tics in childhood.The emerging consensus is that 1-10 children per 1,000 have Tourette's, with several studies supporting a tighter range of 6-8 children per 1,000. Using year 2000 census data, a prevalence range of 1-10 per 1,000 yields an estimate of 53,000-530,000 school-age children with Tourette's in the US, and a prevalence estimate of 10 per 1,000 means that in 2001 about 553,000 people in the UK age 5 or older would have Tourette's. Most cases would be mild and almost unrecognizable in older individuals.

Tourette's is associated with several comorbid conditions, or co-occurring diagnoses, which are often the major source of impairment for an affected child. Among patients whose symptoms are severe enough to warrant referral to specialty Tourette's clinics, only a small minority have no other conditions,and obsessive compulsive disorder (OCD) and attention-deficit hyperactivity disorder (ADHD) are often present. In children with Tourette's, ADHD is associated with functional impairment, disruptive behavior, and tic severity. Other comorbid conditions include self-injurious behaviors (SIB), anxiety, depression, personality disorders, oppositional defiant disorder, and conduct disorders.^[69] One author reports that a ten-year overview of patient records revealed about 40% of patients with Tourette's have "TS-only" or "pure TS", referring to Tourette syndrome in the absence of ADHD, OCD and other disorders.

Tourette syndrome was once thought to be rare: in 1972, the US National Institutes of Health (NIH) believed there were fewer than 100 cases in the United States and a 1973 registry reported only 485 cases worldwide. However, multiple studies published since 2000 have consistently demonstrated that the prevalence is much higher than previously thought. Discrepancies across current and prior prevalence estimates come from several factors: ascertainment bias in earlier samples drawn from clinically referred cases, assessment methods that may fail to detect milder cases, and differences in diagnostic criteria and thresholds.There were few broad-based community studies published before 2000 and until the 1980s, most epidemiological studies of Tourette syndrome were based on individuals referred to tertiary care or specialty clinics. Children with milder symptoms are unlikely to be referred to specialty clinics, so these studies have an inherent bias towards more severe cases. Studies of Tourette syndrome are vulnerable to error because tics vary in intensity and expression, are often intermittent, and are not always recognized by clinicians, patients, family members, friends or teachers; approximately 20% of persons with Tourette syndrome do not recognize that they have tics. Recent studies recognizing that tics may often be undiagnosed and hard to detect use direct classroom observation and multiple informants (parent, teacher, and trained observers), and therefore record more cases than older studies relying on referrals. As the diagnostic threshold and assessment methodology have moved towards recognition of milder cases, the result is an increase in estimated prevalence.

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